In Evolved Universal Terrestrial Radio Access (E-UTRA), a user equipment (UE) requests connection setup through a process referred to as random access. The UE initiates this process by transmitting a random access preamble to an eNodeB (eNB) via a physical random access channel (PRACH). In some circumstances, such as so-called “contention-free” random access, the transmitted preamble may be assigned by the network. Alternatively, in other circumstances such as so-called “contention-based” random access, the transmitted preamble may be randomly selected by the UE from one of multiple possible groups of preambles (e.g. preamble groups A and B).
In contention-based random access, selection of a preamble from a particular group may be used to convey information about the amount of data the UE desires to transmit in subsequent transmissions. For example, selection of a preamble from group A may indicate that the UE desires to transmit a relatively low amount of data, while selection of a preamble from group B may indicate that the UE desires to transmit a relatively high amount of data, or vice versa.
Once a preamble has been transmitted by the UE and detected by the eNB, the eNB transmits a Random Access Response to the UE on a downlink shared channel (DL-SCH). Thereafter, the UE transmits an RRC connection request message to the eNB on an uplink shared channel (UL-SCH). The UE and eNB then communicate using the RRC protocol to establish a connection for data transport between the UE and the network.
Once the UE is connected to the eNB, the eNB facilitates data transport between the UE and the network. In general, the data transport performance may be limited by the radio access capability of the UE, which may be indicated by a UE classification or some other mechanism. Different types of UEs, for instance, may have different radio access capabilities as defined in 3GPP TS 36.306, such as number of receive antennas, maximum number of layers for uplink transmission, maximum data rates in uplink and downlink.
The eNB generally needs to know the UE capabilities in order to properly assign resources, perform control functions, and conduct communication with the UE, among other things. The eNB may obtain information regarding UE capability in any of several ways.
In a typical approach employed in E-UTRA, for example, a UE conveys its capability to the eNB in an RRC UECapabilityInformation message as defined in 3GPP TS 36.331. This information is typically sent upon request from the eNB, after an RRC connection is established.
In an alternative approach, a Category 0 UE, as defined in 3GPP TS 36.306, has restrictions on transport block size, which requires UE capability to be known by the eNB before the RRC connection is established. A Category 0 UE therefore signals its category in the random access procedure, more specifically using a particular logical channel identifier (LCD) in Random Access Msg 3, as defined in 3GPP TS 36.321.
In yet another alternative approach, an even earlier communication of UE capability is proposed. For 3GPP Rel-13, a coverage-limited UE can convey its so-called PRACH repetition level by the random access preamble that it transmits, as described in 3GPP TR 36.888 and 3GPP Tdoc R1-150920. In other words, the UE capability can be identified based on the initial transmission of the random access preamble. To that end the preambles in a cell are partitioned into more groups than preamble groups A and B and the group of preambles for contention-free random access. The network can then apply relevant coverage-enhancement schemes already from the random access response. Coverage-limited UEs may repeatedly transmit the same preamble in several PRACH both to be detected and to be classified by the eNB.